PROCESS FOR STARTING MODE OR STAND-BY MODE OPERATION OF A POWER-TO-GAS UNIT COMPRISING A PLURALITY OF HIGH-TEMPERATURE ELECTROLYSIS (SOEC) OR CO-ELECTROLYSIS REACTORS

Abstract

The application relates to a process for operating in starting mode or in stand-by mode a unit, termed power-to-gas unit, comprising a number N of reactors (1) with a stack of elemental electrolysis cells of solid oxide type (SOEC), the cathodes of which are made of methanation reaction catalyst material(s).

Claims

1. A process for operating in starting mode or in stand-by mode a unit, termed power-to-gas unit, comprising a number N of reactors with a stack of solid oxide (SOEC) type elemental electrolysis cells, the cathodes of which are made of methanation reaction catalyst material(s), wherein, when it is desired to carry out an increase in temperature of the N reactors or of a fraction thereof, or when the level of available electricity is insufficient to carry out a high-temperature electrolysis (HTE) or a co-electrolysis of H.sub.2O and CO.sub.2 within all of the N reactors, the process comprises the following steps: a/ a number P of reactors are supplied with electricity and, if required, with heat, and either steam H.sub.2O, or a mixture of steam and carbon dioxide CO.sub.2 is supplied and distributed to the cathodes of the P reactors so as to carry out, at each cathode of the P reactors, either a high-temperature electrolysis (HTE) of the steam H.sub.2O, or a high-temperature co-electrolysis of steam and carbon dioxide, b/ at least one part of the gases resulting from the electrolysis (hydrogen H.sub.2, steam H.sub.2O ) or from the co-electrolysis (H.sub.2, steam, carbon monoxide CO, carbon dioxide CO.sub.2, methane CH.sub.4) is recovered and is supplied and distributed to each cathode of a number X of reactors not supplied with electricity, the number X being less than or equal to NP, so as to carry out, at each cathode of the X reactors, a methanation by heterogeneous catalysis.

2. The operating process according to claim 1, wherein the carbon dioxide is emitted by a production source, from a production site, in particular chosen from energy production sites (nuclear power stations for the waste heat, biomass methanization or gasification power stations for CO.sub.2-emission and/or waste heat), industrial production sites (cement works, steel industries for CO.sub.2 emission and/or waste heat), service industry buildings such as hospitals, closed-site transportation networks, or else elimination sites, such as waste heat treatment units, etc.

3. The operating process according to claim 1, wherein, in starting mode or in stand-by mode, the heat requirements of the unit, including the reactors (SOEC), are supplied by the waste heat of a production source, of a production site.

4. The operating process according to claim 3, wherein the production site is chosen from energy production sites (nuclear power stations for the waste heat, biomass methanization or gasification power stations for CO.sub.2-emission and/or waste heat), industrial production sites (cement works, steel industries for CO.sub.2 emission and/or waste heat), service industry buildings such as hospitals, closed-site transportation networks, or else elimination sites.

5. The operating process according to claim 1, comprising a step c/ wherein the gases produced by methanation in the SOEC reactors produced by step b/ are introduced into the N dedicated methanation reactors of the power-to-gas unit.

6. The operating process according to claim 1, wherein step a/ is carried out at temperatures of between 600 C. and 1000 C.

7. The operating process according to claim 1, wherein step b/ is carried out at temperatures of between 400 C. and 800 C., and inferior to at least 100 C. to the temperature of step a/.

8. The operating process according to claim 1, wherein steps a/ and b/ are carried out at pressures of between 0 and 100 bar, preferably of between 4 and 80 bar.

9. The operating process according to claim 1, the N reactors all being reactors with a stack of elemental electrolysis cells of SOEC type, each formed from a cathode, an anode and an electrolyte inserted between the cathode and the anode, and a plurality of electrical and fluid interconnectors, each arranged between two adjacent elemental cells with one of its faces in electrical contact with the anode of one of the two elemental cells and the other of its faces in electrical contact with the cathode of the other of the two elemental cells.

10. The operating process according to claim 9, wherein step a/ is carried out by supplying and distributing to each cathode of the P reactors either steam H.sub.2O, or a mixture of steam and carbon dioxide CO.sub.2, or by supplying and distributing steam to each cathode of one of the two adjacent elemental cells of the P reactors and carbon dioxide is fed and distributed to the cathode of the other of the two elemental cells of the P reactors, so as to carry out, at each cathode of the P reactors, either a high-temperature electrolysis of the steam H.sub.2O, or a high-temperature co-electrolysis of steam and of carbon dioxide.

11. The process for producing methane CH4 from an intermittent energy source, implementing the process for operating in stand-by mode a power-to-gas unit according to claim 1, wherein step b/ is carried out when said intermittent source is no longer capable of producing the level of electricity sufficient for all the N reactors.

Description

DETAILED DESCRIPTION

[0058] Other advantages and features of the invention will emerge more clearly on reading the detailed description of examples of implementation of the invention given by way of illustration and in non-limiting manner with reference to the following figures among which:

[0059] FIG. 1 is a diagrammatic view showing the operating principle of a high-temperature water electrolyzer,

[0060] FIG. 2 is an exploded diagrammatic view of a part of a high-temperature steam electrolyzer comprising interconnectors,

[0061] FIG. 3 is a diagrammatic view of the operation in stand-by mode according to the invention of a power-to-gas unit;

[0062] FIG. 4 illustrates, in the form of curves, the change in the degree of conversion to methane with the lowering of the temperature within a non-active reactor of the unit, for a typical mixture of H.sub.2, CO, CO.sub.2 and H.sub.2O at the outlet of an active co-electrolysis reactor of the unit;

[0063] FIG. 5 represents the major composite curve, that is to say the variation in the amounts of heat exchanged in each temperature range and also the energies to be supplied by the utilities, or the exploitation of an externally provided thermal energy, of an example of a power-to-gas unit, in nominal operating mode;

[0064] FIG. 6 represents the major composite curve of the same example of a unit as according to FIG. 5, but in stand-by mode according to the invention.

[0065] Throughout the present application, the terms inlet, outlet, downstream and upstream should be understood with reference to the direction of circulation of the gases from their inlet into the SOEC reactor to their outlet therefrom.

[0066] It is specified that, in FIGS. 1 and 2, the symbols and the arrows for supply of steam H.sub.2O, for distribution and recovery of dihydrogen H.sub.2, of oxygen O.sub.2 and of the current, and of carbon dioxide CO.sub.2, and for distribution and recovery of carbon monoxide CO and of oxygen O.sub.2 and of the current, and of methane CH.sub.4 are shown for the purposes of clarity and precision, in order to illustrate the operation of an SOEC reactor 1 according to the invention.

[0067] It is also specified that all the reactors operating according to steps a/ and b/ of the process according to the method that are described are of solid oxide (SOEC, acronym for Solid Oxide Electrolyzer Cell) type operating at high temperature. Thus, all the constituents (anode/electrolyte/cathode) of an electrolysis cell are ceramic.

[0068] Such constituents may be those of an SOFC fuel cell. The high operating temperature in nominal mode of the reactor 1 during the electrolysis or the co-electrolysis is typically between 600 C. and 1000 C.

[0069] Typically, the characteristics of an elemental SOEC electrolysis cell suitable for the invention, of the cathode-supported cell (CSC) type, may be those indicated as follows in Table 2 below.

TABLE-US-00001 TABLE 2 Electrolysis cell Unit Value Cathode 2 Constituent material NiYSZ Thickness m 315 Thermal conductivity W m.sup.1 K.sup.1 13.1 Electricity conductivity .sup.1 m.sup.1 10.sup.5 Porosity 0.37 Permeability m.sup.2 .sup.10.sup.13 Tortuosity 4 Current density A .Math. m.sup.2 5300 Anode 4 Constituent material LSM Thickness m 20 Thermal conductivity W m.sup.1 K.sup.1 9.6 Electricity conductivity .sup.1 m.sup.1 .sup.1 10.sup.4 Porosity 0.37 Permeability m.sup.2 .sup.10.sup.13 Tortuosity 4 Current density A .Math. m.sup.2 2000 Electrolyte 3 Constituent material YSZ Thickness m 5 Resistivity m 0.42

[0070] A water electrolyzer is an electrochemical device for producing hydrogen (and oxygen) under the effect of an electric current.

[0071] In HTE high-temperature electrolyzers, the electrolysis of water at high temperature is carried out using steam. The function of an HTE high-temperature electrolyzer is to convert the steam into hydrogen and oxygen according to the following reaction:

2H.sub.2O.fwdarw.2H.sub.2+O.sub.2.

[0072] This reaction is carried out electrochemically in the cells of the electrolyzer. As represented diagrammatically in FIG. 1, each elemental electrolysis cell 1 is formed from a cathode 2 and an anode 4, placed on either side of a solid electrolyte 3. The two electrodes (cathode and anode) 2, 4 are electron conductors, made of porous material, and the electrolyte 3 is gas-tight, an electron insulator and an ion conductor. The electrolyte may in particular be an anionic conductor, more specifically an anionic conductor of O.sup.2 ions and the electrolyzer is then referred to as an anionic electrolyzer.

[0073] The electrochemical reactions take place at the interface between each of the electron conductors and the ionic conductor.

[0074] At the cathode 2, the half-reaction is the following:

2H.sub.2O+4e.sup..fwdarw.2H.sub.2+2O.sup.2.

[0075] At the anode 4, the half-reaction is the following:

2O.sup.2.fwdarw.O.sub.n+4e.sup..

[0076] The electrolyte 3 inserted between the two electrodes 2, 4 is the site of migration of the O.sup.2, ions under the effect of the electric field created by the potential difference imposed between the anode 4 and the cathode 2.

[0077] As illustrated in parentheses in FIG. 1, the steam at the cathode inlet can be accompanied by hydrogen H.sub.2 and the hydrogen produced and recovered at the outlet may be accompanied by steam. Likewise, as illustrated with dashed lines, a draining gas, such as air, may also be injected at the inlet in order to remove the oxygen produced. The injection of a draining gas has the additional function of acting as a heat regulator.

[0078] An elemental electrolysis reactor consists of an elemental cell as described above, with a cathode 2, an electrolyte 3, and an anode 4, and of two mono-polar connectors which ensure the electrical, hydraulic and thermal functions.

[0079] To increase the flow rates of hydrogen and oxygen produced, it is known practice to stack several elemental electrolysis cells on top of one another, separating them with interconnection devices, usually known as interconnectors or bipolar interconnecting plates. The assembly is positioned between two end interconnecting plates which support the electrical feeds and gas feeds of the electrolyzer (electrolysis reactor).

[0080] A high-temperature water electrolyzer (HTE) thus comprises at least one, generally a plurality of, electrolysis cells stacked on top of each other, each elemental cell being formed from an electrolyte, a cathode and an anode, the electrolyte being inserted between the anode and the cathode.

[0081] The fluid and electrical interconnection devices that are in electrical contact with one or more electrodes generally perform the functions of conveying and collecting electric current and delimit one or more gas circulation compartments.

[0082] Thus, a cathode compartment has the function of distributing the electrical current and steam and also recovering hydrogen at the cathode in contact.

[0083] An anode compartment has the function of distributing the electric current and also recovering the oxygen produced at the anode in contact, optionally using a draining gas.

[0084] FIG. 2 represents an exploded view of elemental units of a high-temperature steam electrolyzer according to the prior art. This HTE electrolyzer comprises a plurality of elemental electrolysis cells C1, C2, of solid oxide (SOEC) type stacked alternately with interconnectors 5. Each cell C1, C2 . . . consists of a cathode 2.1, 2.2, . . . and of an anode 4.1, 4.2, between which is placed an electrolyte 3.1, 3.2 . . . .

[0085] The interconnector 5 is a component made of metal alloy which ensures separation between the cathode compartment 50 and the anode compartment 51, defined by the volumes between the interconnector 5 and the adjacent anode 4.2 and between the interconnector 5 and the adjacent cathode 2.1, respectively. It also ensures the distribution of the gases to the cells. The injection of steam into each elemental unit takes place in the cathode compartment 50. The collecting of the hydrogen produced and of the residual steam at the cathode 2.1, 2.2 . . . is performed in the cathode compartment 50 downstream of the cell C1, C2 . . . after dissociation of the steam by the latter. The collecting of the oxygen produced at the anode 4.2 is performed in the anode compartment 51 downstream of the cell C 1, C2 . . . after dissociation of the steam by the latter.

[0086] The interconnector 5 ensures the passage of the current between the cells C1 and C2 by direct contact with the adjacent electrodes, i.e. between the anode 4.2 and the cathode 2.1.

[0087] In the HTE high-temperature co-electrolyzers, the high-temperature co-electrolysis is carried out using steam and carbon dioxide CO.sub.2. The function of an SOEC high-temperature co-electrolyzer is to convert the steam and the CO.sub.2 into hydrogen, carbon monoxide and oxygen according to the following reaction:

CO.sub.2+H.sub.2O.fwdarw.CO+H.sub.2+O.sub.2.

[0088] A co-electrolyzer 1 may comprise exactly the same solid oxide constituents (SOEC) as an HTE electrolyzer that has just been described. Usually, the steam and the carbon dioxide CO.sub.2 are mixed, in ratios chosen according to the application, before entry into the co-electrolyzer and injected simultaneously into each cathode compartment 50.

[0089] Represented in FIG. 3 is a power-to-gas unit U which uses a large number X and P of co-electrolysis reactors upstream of a plurality of dedicated methanation reactors 6.

[0090] This FIG. 3 illustrates an example of the process for operating this unit U in stand-by mode according to the invention: in this case, a number P of reactors 1 is supplied with electricity and operates in co-electrolysis mode, while the resulting X reactors, which are not supplied with electricity, are supplied with a portion of the gas products resulting from the co-electrolysis of the P reactors.

[0091] The example given corresponds to a power-to-gas unit with a co-electrolysis power of 20 MWe in nominal operation, which is coupled to an industrial source, typically a steel industry which emits CO.sub.2 and a heat of medium temperature at around 200 C.

[0092] For this 20 MWe co-electrolysis power and under the chosen operating conditions of an SOEC reactor, i.e. with a heat-neutral operation at a voltage of 1.33 V, a current density of 1.1 A/cm.sup.2 and under a pressure of 10 bar, the power-to-gas unit U comprises a number N equal to 1855 SOEC reactors 1, which corresponds to a stack of 50 cells C1, C2 . . . , each of 196 cm.sup.2.

[0093] This number N of 1855 reactors is grouped together in 53 chambers of 35 reactors each.

[0094] The nominal operation, i.e. with a 100% load, of the power-to-gas unit comprises the following steps: [0095] a step of 20 MWe co-electrolysis by all of the 1855 SOEC reactors operating in parallel, supplied with steam and carbon dioxide CO.sub.2, from a capture upstream at the outlet of the industrial source, at high temperature (800 C.) and producing a mixture of H.sub.2, CO, H.sub.2O and CO.sub.2; [0096] a step of partial condensation of the steam at the outlet of the co-electrolysis reactors; [0097] a passage through one of several (in parallel) first dedicated catalytic methanation reactor(s) 6; [0098] after a step of intermediate condensation of the steam, passage through one of several (in parallel) second dedicated catalytic methanation reactor(s) 6, which finalize(s) the reaction for methanation of H.sub.2, CO and CO.sub.2 so as to produce therefrom methane CH4; [0099] a step of more thorough condensation of the water so as to obtain a synthesis methane with injection specifications for the natural gas network.

[0100] In addition, in nominal operation, the power-to-gas unit U has a thermal integration according to which the heat from the methanation reactors 6 is in particular fully used to vaporize the water at the inlet of the co-electrolysis reactors 1.

[0101] The supply of electricity to the unit U is ensured by a surplus of electricity production from renewable energies. This surplus has been estimated at 1800 h/year with a forecast of 10 years.

[0102] Working with a hypothesis of nominal operation of the unit only during surplus hours, long periods of stand-by should thus be envisioned over the year, during which the electrical needs are supplied by purchase from the electrical network.

[0103] The operating in stand-by mode according to the invention, as illustrated in FIG. 3, consists in carrying out the following steps:

[0104] a/ a number P of reactors of the unit U is supplied with electricity and either steam H.sub.2O, or a mixture of steam and carbon dioxide CO.sub.2, is supplied and distributed to the cathodes of the P reactors, so as to carry out, at each cathode of the P reactors, either a high-temperature electrolysis of the steam H.sub.2O, or a high-temperature co-electrolysis of steam and carbon dioxide,

[0105] b/ at least one part of the gases resulting from the electrolysis (hydrogen H.sub.2, steam H.sub.2O ) or the co-electrolysis (H.sub.2, steam, carbon monoxide CO, carbon dioxide CO.sub.2, methane CH.sub.4) is recovered and is supplied and distributed to each cathode of a number X of reactors not supplied with electricity, of the unit U, the number X being less than or equal to N-P, so as to carry out, at each cathode of the X reactors, a methanation by heterogeneous catalysis.

[0106] By applying the steps according to the invention, a heat management strategy in stand-by mode has been implemented as follows: [0107] evaluation of the heat losses at 4.64 kWth per chamber containing 35 reactors in stand-by; [0108] the products from the P reactors are injected into the other X SOEC reactors, so as to enable an internal methanation according to the following exothermic reactions, thermodynamically promoted by a lowering of the temperature:

CO.sub.2+4H.sub.2.fwdarw.CH.sub.4+2H.sub.2O .

CO+3H.sub.2.fwdarw.CH.sub.4+H.sub.2O.

An example of change in methane production under 5 bar of pressure is shown in FIG. 4; [0109] a thermodynamic calculation of the shift in the methanation reaction equilibrium between the active P reactors (800 C.) and the inactive X reactors (600 C.) has shown that an active co-electrolysis reactor at 800 C. under a pressure of 10 bar makes it possible to maintain the temperature of seven SOEC reactors at 600 C. without supply of electricity, that is to say operating of the unit U at only 12.5% load.

[0110] This operating strategy in stand-by mode according to the invention thus clearly makes it possible to maintain at high temperature all the N SOEC reactors, in return for which there is a consumption of electricity of only 12.5% of the nominal operation, i.e. P=12.5% of the number N with X=NP.

[0111] As shown in FIG. 3, once used for keeping the X inactive SOEC reactors hot, the reagents finalize their methanation in the dedicated methanation reactors 6, as for the nominal operation.

[0112] The power-to-gas unit U according to the invention thus continues to implement the exploitation of the electricity to synthesis methane, but at a reduced load.

[0113] The inventors have carried out a thermal integration of a unit U, by means of the ProSimPlus commercial modelling software, in order to carry out an overall heat balance in the two operating modes, namely nominal operating and operating in stand-by mode according to the invention.

[0114] This balance is illustrated respectively in FIG. 5 (nominal operating mode) and in FIG. 6 (stand-by mode according to the invention). It is specified that, according to their usual definitions, the cold utilities are the cooling fluids used for discharging the excess heat from the unit typically at low temperature, while the hot utilities are the heating additions used to meet the heat requirements of the unit typically at medium or high temperature.

[0115] From the curves illustrated in FIGS. 5 and 6, the heat balance shows: [0116] cold utility needs for the unit, which are by assumption supplied through consumptions of electricity, for example from an air-cooled exchanger or a heat pump; [0117] hot utility needs, which are however limited to a temperature close to 200 C.: [0118] for the operating in nominal mode, which is normal; [0119] likewise for the operating in stand-by mode, by virtue of the heat strategy deployed, this being despite the maintaining at high temperature and the heat losses on the X SOEC reactors which are not supplied with electricity.

[0120] If the unit is coupled with an industrial source which emits both CO.sub.2 and residual/excess heat at medium temperature, this need for hot utilities at medium temperature does not have to be compatibilized in the electrical balance of the unit.

[0121] The results of the thermal calculation by the ProSimPlus software, and also of the electrical yield of a power-to-gas unit are illustrated in Table 1 below, for the nominal and stand-by operating modes in accordance with the invention with a load of 12.5% of the nominal.

TABLE-US-00002 TABLE 1 Stand-by according Operating mode Nominal to the invention Electricity (MW) 24.59 3.07 including cold utility Water consumption (t/h) 2.56 0.32 Methane CH.sub.4 production (t/h) 1.11 0.14 Oxygen production (t/h) 4.49 0.56 Yield (%) 69.1 69.1

[0122] From this Table 1, it emerges that the electrical yield of the unit U is maintained at partial load, despite the heat requirements for maintaining at high temperature the X reactors which are not supplied with electricity.

[0123] The invention is not limited to the examples which have just been described; features of the examples illustrated in variants that are not illustrated may in particular be combined with one another.

REFERENCES CITED

[0124] [1]: Etude portant sur l'hydrogne et la mthanation comme procd de valorisation de l'lectricit excdentaire [Study relating to hydrogen and methanation as a process for exploiting excess electricity]ADEME, GRTgaz, GrDF; September 2014; [0125] [2]: Sensitivities of Power-to-Gas within an optimized Energy System; E. Kotter; IRES-2015 [0126] [3]: Energy Storage for a Greenhouse Gas Neutral Society: Demand and Long-term Strategy; M. Nowakowski; IRES-2015